The invention relates to a CRT (cathode ray tube) with a colour selection electrode having rows of elongated openings. The colour selection electrode is also called a xe2x80x98maskxe2x80x99. More in particular it relates to a CRT with a tensioned colour selection electrode in which electrode tension is applied in one direction and the colour selection electrode comprises elongated openings in said direction, the openings being separated from each other in said direction by bridges, the dimension of said openings in said direction being more than 5 mm. Basically such a design is half-way in between two conventional designs, the conventional slotted shadow mask, in which many small (typically smaller than 1.5 mm) elongated or round holes are made, and the so/called tension or xe2x80x98slitxe2x80x99 mask, in which a large number of strips are put under tension. The strips are throughout their length (typically 20 or more centimeter) separated by slits. Both of the conventional designs have shortcomings, the conventional shadow mask absorbs much of the electrons, reducing the intensity of the image, whereas the xe2x80x98slit maskxe2x80x99 is very sensitive for microphony and difficult to handle.
A CRT of the in the opening paragraph described type is known from U.S. Pat. No. 4,942,332.
In U.S. Pat. No. 4,942,332 a slit-type flat foil tension mask is described having slits with large (in comparison to conventional masks) longitudinal dimensions. Typically the length of the slits is of the order of 1 inch.
The bridges in between the slits provide mechanical strength to the tensioned colour selection electrode, without substantially reducing the image brightness or deforming the mask when the mask is put under tension. However, they also pose a problem in that the bridges may be visible as two straight horizontal lines, reducing the image quality.
In U.S. Pat. No. 4,942,332 two possible solutions for this problem are described, one being constituted by randomising the length (pitch) of the slits and another being constituted by introducing false bridges in the colour selection electrode.
The latter solution (false bridges) has the serious draw-back that these false bridges intercept the electrons, thus reducing the image intensity. Instead of a small number clearly visible lines a large number of lines are introduced. They are not individually visible but reduce the intensity of the image substantially.
The first solution (randomising) has been found by the inventors to result in negative effects on the image. The bridges are in many instances visible despite the randomisation and the image obtains a xe2x80x98patchyxe2x80x99 appearance.
It is an object of the invention to provide a cathode ray tube of the in the first paragraph described type in which the visibility of the bridges is reduced without reducing substantially the image intensity, and without getting a patchy appearance or at least reducing the chance for such an appearance.
To this end the cathode ray tube in accordance with the invention is characterised in that the bridges are formed in a regular repetitive pattern, wherein the pattern seen in a direction transverse to the longitudinal direction substantially repeats itself after n rows of slotted openings, the shift in a direction along the openings between adjacent rows being a multiple of approximately a distance av/n, and wherein n is greater than 4 and wherein av/dhxe2x89xa6n2xe2x89xa68av/dh where av is the vertical pitch of the openings and dh is the horizontal pitch between rows and wherein rows in which the respective patterns of openings are shifted in the direction along the elongated openings by av/n or xe2x88x92av/n are more than n/4 rows apart.
Instead of randomising the pattern of bridges is made repetitive with a high number order of repetition (n). The length of the openings is considerably larger then for standard shadow masks (avxe2x89xa75 mm).
The inventors have realised that randomisation of the position of the bridges as described in U.S. Pat. No. 4,942,332 leads to unexpected problems. At some parts of the image the bridges become visible, namely there where a number of bridges happen to be at the same horizontal position, or where adjacent bridge happen to be spaced at such distances that Moirxc3xa9 effects occur, where at other parts they are not. The randomisation in fact does not change the distance between the bridges seen in a horizontal dimension by a large amount. The randomisation as described in U.S. Pat. No. 4,942,332 is for example 0.02xe2x80x3, which means that on average the distance between adjacent bridges is 0.01xe2x80x3 or 0.25 mm, and because of the randomisation the distance is sometimes much less. This still leads to lines being visible in the image, to which lines the human eye is very sensitive. Randomising invariantly also leads to clustering, resulting being visible in parts of the image. The human eye is very sensitive to such irregularities in the image. The image is perceived as xe2x80x98patchyxe2x80x99 because of this effect. In some sense this problem is a greater problem than a straight line being visible. The straight line is always there and it is a problem that the viewer understands and will most likely be at least to some degree be visible when the device is bought and will effect all modes of image reproduction in more or less the same degree. The xe2x80x98patchyxe2x80x99 image due to randomisation is something that is dependent of the image that is displayed, and also on the particular mode (VGA, UGA, XVGA etc) with which the image is displayed. These problems become usually visible at higher resolution of the image, i.e. the higher the image quality. Such problems manifest themselves more often than not after sale and effect in particular the high quality image modes, leading to the clearly unwanted effect that the xe2x80x98lower image qualityxe2x80x99 in fact gives a higher quality image than the xe2x80x98high quality imagexe2x80x99.
Repetitive patterns, if of high enough symmetry (n greater than 4) are much less visible then the two lines of the prior art. In fact the xe2x80x98intensityxe2x80x99 of the straight lines are reduced by at least a factor 2.5, which is sufficient for most purposes. Furthermore, because the pattern is regular the effect of the bridges on the image is evenly distributed, and there will be no xe2x80x98patchesxe2x80x99 in the image. There may still, however, occur visible lines in the image. The openings are shifted in respect of other rows by an amount of approximately av/n or a multiple thereof (2av/n, 3av/n, 4av/n, . . . (nxe2x88x921)av/n). It is remarked that a shift of (nxe2x88x921)av/n is the same as a shift of xe2x88x92av/n, a shift of (nxe2x88x922)av/n is the same as xe2x88x922av/n etc. The condition av/dhxe2x89xa6n2xe2x89xa68av/dh and the condition that the rows in which the respective patterns of openings are shifted in the direction along the elongated openings by approximately av/n or xe2x88x92av/n are spaced apart further than n/4 rows in a direction perpendicular to elongated openings, reduce the occurrence and visibility of such lines to such a large degree that they are not or hardly visible.
Preferably
av/dhxe2x89xa6n2xe2x89xa64av/dh, even more preferably
2av/dhxe2x89xa6n2xe2x89xa64av/dh, most preferably
n2≈2av/dh. (i.e. differing less than 25% from each other)
These ranges gives the best results.
Preferably the rows in which the respective patterns of openings are shifted in the direction along the elongated openings by av/n or xe2x88x92av/n are spaced apart n/3 to n/2 (rounded off to the nearest whole number) rows in a direction perpendicular to elongated openings.
It is remarked that within the concept of the invention a small tilt of a few degrees of the pattern as a whole is possible. Strictly speaking, when a pattern repeats itself after n rows, for instance after 10 rows with a value of av of 10 mm, and a value of dh of 0.22 mm, and in addition a small tilt of for instance 5 degrees is added, the pattern as seen along the line of tilt repeats itself after 10 rows, but strictly speaking seen along a horizontal line, the pattern, because of the small tilt, repeats itself after a much larger number of rows (or not at all). For instance a tilt of 5 degrees will make the pattern repeat itself after some 50 mm in the horizontal direction i.e. after some 225 rows. Patterns with, a small tilt, leading to patterns which, but for the tilt, repeat themselves after n rows fall within the scope of the invention. Likewise patterns which include a gradual change in the value of av, which would introduce effects similar to a small tilt, fall within the scope of the invention. In general if the effects of such higher order additions are, within the basic repetitive pattern of n rows, less than approximately 0.25av/n (in horizontal direction) and preferably less than 0.10av/n the resulting patterns are within the scope of the invention.